Apparatus and methods for producing film polymers based on p-xylylene and made by vapor-phase polymerization are known in the art. A typical vapor-phase deposition apparatus comprises a vaporizer, a pyrolizer, a deposition chamber, a condenser and a vacuum pump connected in series. The p-xylylene film coating process is carried out by vaporizing the substituted or unsubstituted cyclic dimer, di-p-xylylene, which is fed as a powder into the vaporization chamber. The dimer vapor is conducted into the pyrolizer where the dimer is cleaved to obtain the reactive diradicals. The diradicals or monomer vapor is passed into the deposition chamber where the vapor is condensed on a substrate or workpiece to form the p-xylylene film coating.
Unreacted or unpyrolized dimer vapor together with contained impurities pass through the deposition chamber into the condenser. The vacuum pump is used to draw the vapor through the system and to maintain the desired pressure conditions.
When the polymerization rate of the p-xylylene is too fast so that the monomer conversion is incomplete, the physical properties of the polymeric film are adversely affected. The tensile strength and insulating properties are lower than expected of the p-xylylene molecule. In addition, the film displays a nonuniform density and diminished resistance to molecular liquid and gas diffusion.
Attempts have been made in the past to regulate the temperature of the vaporizer in order to control the polymerization rate and thereby form a film coating having the desired physical properties. A conventional procedure has been to operate the apparatus with a set vaporizer temperature. This procedure frequently resulted in the formation of layers of polymer film having different and nonuniform physical properties. Another associated problem was simply that of setting the vaporizer temperature at a level which would result in the optimum polymerization rate. When it was observed from the formation of the film that the polymerization rate was too fast, the temperature of the vaporizer was lowered. The lowered temperature often required an unreasonably long time in order to produce a film of the desired thickness.